A method for controlling a gas turbine, which allows the operator to operate a gas turbine power station safely, is normally referred to as an operating concept. The disclosure relates to the optimization of load operation, that is to say that part of an operating concept which controls the operation when the gas turbine is connected to an electrical grid system, and is delivering power to it. The operating concept defines how various parameters of the gas turbine must be controlled for safe operation of the gas turbine. The operating concept is implemented by the controller. By way of example, the gas turbine power can be adjusted by varying the at least one turbine inlet temperature, the compressor inlet mass flow, or both parameters. By way of example, the compressor inlet mass flow can be adjusted by varying the inlet geometry of the compressor by means of a variable inlet guide grid.
The turbine inlet temperature level essentially governs service life consumption and the length of the inspection interval of the gas turbine. Furthermore, it essentially governs the exhaust gas emissions from the gas turbine.
If the inlet mass flow is constant, the power of a gas turbine is governed essentially by the turbine inlet temperature level. The gas turbine outlet temperature is proportional to the turbine inlet temperature level, and is inversely proportional to the pressure ratio of the gas turbine.
The efficiency of a combined gas and steam turbine power station in a so-called combined-cycle power station is proportional to the gas turbine outlet temperature level and the gas turbine efficiency. In consequence, the overall efficiency and the power of a combined-cycle power station are proportional to the gas turbine inlet temperature.
In the theoretical Brayton cycle, with a constant turbine inlet temperature and constant component efficiencies, the efficiency of the gas turbine is proportional to the pressure ratio. In the actual machine, when the turbine inlet temperature is constant, the pressure ratio is proportional to the mass flow. However, in reality, the component efficiencies vary as a function of the mass flow and the temperature. In particular, the compressor efficiency is dependent on the mass flow, which is controlled by the inlet guide vane setting. Furthermore, for example, losses in inlets and outlets or diffusers are a function of the volume flow or mass flow. A corresponding situation applies to the boiler, to its pressure loss for the hot gases flowing through it, and to the water-steam circuit that is connected. Designing the components for a mass flow means that the efficiency of an actual gas turbine and of an actual gas turbine combined-cycle power station does not rise in proportion to the pressure or mass flow, but has a maximum. If the mass flow, and therefore the pressure ratio, are raised above this maximum, then the efficiency falls. In general, it is possible to increase the power of the power station above the maximum efficiency operating point.
In the upper load range, and when a load reduction takes place starting from full load (or base load), for example, modern gas turbines are controlled such that the turbine inlet temperature (or hot gas temperature), which is limited by a limiter, is first of all reduced. The limiter is correspondingly reduced from the full load value to the part load value. The inlet mass flow is then reduced by controlling the pitch angle of at least one variable row of inlet guide vanes VIGV in the compressor, which is itself limited by a limit. The fuel mass flow is reduced during reduction in the inlet mass flow, in order to control the turbine inlet temperature below the respectively applicable limit. As soon as the turbine outlet temperature TAT, which rises in inverse proportion to the falling pressure ratio as the mass flow decreases at a constant turbine inlet temperature TIT, has reached the relevant TAT limit value, the fuel mass flow is reduced in order to control the TAT below the limit. The TIT then falls below its limit. EP0718470 discloses an example of an operating concept for a modern gas turbine with sequential combustion.
In order to operate a gas turbine in accordance with the concept described in EP0718470, a TIT must be determined, or must be determined approximately. Various temperatures can be used as the turbine inlet temperature. It is possible to use a theoretical mixture temperature of the hot gases and of all the cooling air mass flows in accordance with ISO 2314/1989. However, for example, the control process can also be based on the hot gas temperature upstream of the inlet to the turbine, or the so-called “firing temperature”, a mixing temperature after the first turbine inlet guide vane.
EP1840354 discloses one example for determining the TIT. Further, usually simpler but less precise approximations are known to the person skilled in the art.
Conventionally, electricity generating costs can be minimized, in the sense of flexible adaptation of the gas turbine inlet temperature and/or the position of the compressor inlet grid only on starting operation again, when the appropriate limits for part load and full load to achieve specific maximum or minimum load values are defined. For example, it will be necessary to start up the gas turbine again if, in the event of an increase in the fuel price, one wished to achieve the efficiency by accepting a reduced service life and therefore a shorter servicing interval by increasing the temperature limits.